Managing power in capacitive touch sensing applications, Part 2

Capacitive sensing enabled user interfaces is a widely accepted technology that often replaces existing mechanical switches and push buttons. Capacitive user interfaces are also used in battery-powered, handheld, and portable electronic devices. This penetration into portable and handheld electronics which mandate a long battery life and the constant focus on "green" technology has increased the importance of low-power applications.

This series of articles will feature various tips and techniques which will help you build a low-power capacitive user interface panel. The first part of this article discussed the following four techniques:

Following layout best practices to optimize the sensor parasitic capacitance (CP)

Using sleep mode and optimizing the report rate of the capacitive sensing controller

Optimizing the report rate based on finger touch events

Using the priority rule to wake up the capacitive controller from sleep mode

This part focuses on the next three techniques that can be used to optimize power consumption:

Ganged capacitive sensor model to wake up the capacitive controller from standby mode

Use of a proximity sensor to wake up the capacitive controller from standby mode

Use of an external regulator to turn off the power to the user interface unit

Ganged capacitive sensor model to wake up capacitive controller from standby mode
The priority rule (discussed in Part 1) is useful only when one or a few specific sensors are used to wake the system up from standby mode. In most other cases, the system would be required to wake up when any of the sensors are activated. Typically, the greater the number of buttons that can wake up the system, the greater the average power consumed.

Ganged sensor sampling can be used to address this problem without increasing the average power. In this method, during the standby mode of the system, all of the physical sensors which can wake up the system are connected together to form a single virtual ganged sensor in the design. Scanning only the ganged sensor consumes lesser time than scanning all of the sensors; therefore, the capacitive controller can be in sleep mode for a longer time thereby reducing the average power consumed. (Refer to using sleep mode in Part 1 for more details).

If any of the physical sensors are touched, the sensor capacitance of the ganged sensor increases and a touch is detected. However, the specific button that was touched during the standby mode cannot be determined while sensing a touch on the ganged sensor.

To detect the button that was touched during standby mode, the capacitive controller should wake up and move into its active mode. The physical sensors need to be disconnected from the ganged sensor and scanned individually to identify the touched sensor.

In this method, the ganged sensor helps to optimize the average power by combining multiple physical sensors into a single virtual ganged sensor which ensures the capacitive controller reverts to active more only when a touch is detected. If after the capacitive controller moves to active mode, no finger touch is detected on the user interface panel for a certain period of time, then the capacitive controller should revert back to the ganged sensor mode.

The average system current consumption using this method is similar to that of the priority rule wake up method but provides the system the capability to wake up from a touch to any of the buttons.

Programmable capacitive controllers such as Cypress' CapSense and CapSensePLUS controllers help to dynamically connect multiple sensors together to form a ganged sensor that optimizes the average power.